1. Field of the Invention
The invention relates to a method of measurement of the level of the surface of a bath of molten metal beneath a fluid layer of slag in a metallurgical vessel, using a detector which is moved through the slag layer into the metal bath and then withdrawn. The method is especially applicable in steel making.
2. Description of the Prior Art
A method as described above is known from NL-A-8005997.
When carrying out metallurgical processes in a metallurgical vessel, such as a furnace, ladle or tundish, it is often important to have accurate knowledge about the height of the bath surface of the metal, for example in order to obtain an impression of the wear of a refractory lining of a furnace. Knowledge about this bath level may also be important if a blowing lance, which is moved to just above the bath surface, is being operated for example for blowing oxygen onto the bath.
In steel making, typically a steel ladle is first filled with a quantity of scrap onto which pig iron is then cast. Then oxygen is blown onto the metal bath via a blowing lance for about twenty minutes. After the blow-period an intermediate pause is introduced, in which several measurements are made on the metal bath, including determination of the carbon content. These measurements are carried out using a so-called sublance with a detector attached to it, in order to bring the detector below the fluid layer of slag floating on the metal bath. When the measurements are completed, the sublance is withdrawn and another relatively short blow-period usually follows in order to obtain the desired quality of the metal bath, which has become steel. The duration of this shorter, second blow-period depends on at least one of the measurements carried out. At the end of the second blow-period, a final pause may occur during which it is usual again to move the sublance with a detector into the steel ladle in order to check the result achieved.
The measurements carried out during the intermediate pause are limited to the measurement of temperature, carbon content and to taking a steel sample for analysis purposes. The measurements carried out during the final pause also relate to temperature measurement and steel sample taking. In that case, however, assessing the carbon content may be replaced by oxygen measurement, on account of the accurate information concerning carbon content in low-carbon steel which this provides.
Only once in each 8 hours, that is to say every time the shift changes in the 24-hour per day operation, a bath height measurement is also carried out. To that end, shortly after the final pause measurement the sublance is brought into the steel ladle again with a detector which is suitable for measuring the bath level. NL-A-8005997 describes a resistance sensor suitable for this.
The reliability of this bath height level measurement is increased by combining the resistance measurement with a temperature measurement. In fact a temperature jump occurs when the detector is moved out of the metal bath into the slag. The resistance increase and the temperature increase should in principle form a good indication of the boundary between steel and slag.
However it is not always possible to determine reliably this temperature jump. In the event that, on final pause measurement it is found that the final temperature of the steel is too high, coolant scrap must be added for cooling. However, this results in disappearance of the temperature profile, as described, with a jump in temperature between steel and slag. In that case the result of the bath level measurement must be based only on the measurement of the change in resistance, which itself provides an uncertain measurement.
AU-B-514863 also describes measurement of the height of a steel bath in a converter or other vessel by insertion and withdrawal of a probe. An electrode is lowered through the slag layer to make electrical contact with the metal bath, completing a circuit. The voltage due to resistance of this circuit, drops when the contact is made, this giving the position of the metal surface. This measurement also requires a special step in the steel-making process.